This invention relates to a method of testing a set of wheel speed sensors, included in an anti-lock brake control system for motor vehicles to produce signals indicative of the speeds of the vehicle wheels and thereby provide for proper functioning and proper electrical connections.
Hydraulic brake systems for motor vehicles are usually equipped with some form of anti-lock brake control system, or antiskid system, in order to prevent wheel lockup during braking. Should the brakes lock the vehicle wheels, they would begin to skid. Generally, in hydraulic brake systems, brake fluid is sent from a master cylinder to wheel cylinders at respective vehicle wheels upon application of a brake pedal. The wheel cylinders act to slow or stop the revolving wheels, which in turn slow or stop the vehicle.
Incorporated with the hydraulic brake system, the anti-lock brake control system functions to stop the supply of the brake fluid to the wheel cylinders, or to release the fluid pressure in the wheel cylinders, as required by the deceleration of the vehicle wheels or by the decrease of the wheel speed in relation to vehicle velocity. A set of wheel speed sensors are provided at each vehicle wheel for providing signals representative of the wheel speed. The wheel speed will increase again and come close to the vehicle velocity as a result of interaction will the road after the fluid pressure in the wheel cylinders is released. Thereupon, the control system will again permit delivery of the brake fluid to the wheel cylinders. The same cycle of brake pressure control is repeated thereafter as required for the most efficient braking.
Japanese Utility Model Laid-Open Publication No. 62-20367 proposes a trouble detection system for the wheel speed sensors of the anti-lock brake control system. This prior art system detects troubles associated with the wheel speed sensors when the wheel speed represented by the output from any wheel speed sensor drops and remains below a predetermined limit for a certain length of time while the vehicle is not braked. The trouble thus detected is, however, limited to disconnection, due to vehicle vibration, of the electrical connections between the wheel speed sensors and the logic controller of the anti-lock brake control system, or to the destruction of the tone wheel assemblies associated with the wheel speed sensors.
The wheel speed sensors must also be checked to see if they are connected to the correct terminals of the logic control circuit. If not, the control circuit would make incorrect interpretations of the incoming wheel speed signals and incorrectly control the wheel brakes. The conventional practice has been to use oscilloscopes or other testers external to the vehicle for detecting possible misconnections between the sensors and the control circuit.
There is another important test that must be conducted on wheel speed sensors. The anti-lock brake control system in general must function to prevent wheel lockup until immediately before the vehicle comes to a standstill after braking. The lowest vehicle speed at which the brake control system must function may, for example, be 2.75 kilometers per hour (km/h). Wheel speed sensors must be tested to make sure that they produce the proper signals at this lowest vehicle speed. This test has heretofore been made on wheel speed sensors before they are mounted in place on motor vehicles.